1. Field of the Invention
The present invention relates to transmission mechanisms for vehicles having an HST (hydrostatic transmission), and more particularly to a transmission mechanism for work vehicles equipped with a work device externally of the vehicle body, and also to a pressure oil supply system for the mechanism.
2. Description of the Related Technology
It is commonly demanded that various vehicles be shortened in forward or rearward length to diminish the turning radius, improve the ease of driving and achieve cost reductions. Especially, this is strongly demanded of work vehicles which are equipped with a work device externally of the vehicle body in front or in the rear thereof.
Work vehicles provided with a work device externally of the vehicle body have a PTO shaft at a portion of the transmission for transmitting power to the work device. With such work vehicles, it is desired that the transmission shaft interconnecting the PTO shaft and the input shaft of the work device be disposed horizontally to the greatest possible extent in order to prevent the joints of the transmission shaft from giving off noise and assure the joints of improved durability. For example, JP-U1 No. 56555/1993 and JP-A No. 204135/1990 disclose transmissions for work vehicles intended to meet this demand. The disclosed arrangements include pulleys or like cooperative means for positioning a PTO shaft 1103 at a lower level than a front axle 1105 to cause the shaft to project from the front end face of the front axle case (as indicated at X in FIG. 19). Generally, the transmission shaft 1104 for a work device 1106 needs to be lengthened (as indicated at Y in FIG. 19) if it is attempted to position the transmission shaft 1104 closer to a horizontal while causing the PTO shaft 1103, as positioned approximately at the same level as the engine drive shaft 1101a, to project from the front end of the front axle case, consequently increasing the overall vehicle length. Alternatively, the transmission shaft 1104 must be inclined (as indicated at Z in FIG. 19) if an attempt is made to shorten the overall vehicle length while causing the PTO shaft 1103 to project from the front end of the front axle case at substantially the same level as the engine drive shaft 1101a, consequently increasing the load on the transmission shaft and impairing the durability of the transmission shaft. In view of these structural drawbacks, the transmissions disclosed in the foregoing publications are so constructed as stated above.
In the case of the conventional construction described, however, a differential gear unit is to be interposed between the engine output shaft (or HST motor shaft) and the PTO shaft, so that the cooperative mechanism between the two shafts must be adapted to avoid the interference with the differential gear unit. This results in an increase in the vehicle cost and an impaired maintenance work efficiency.
With vehicles, especially with work vehicles, it is known to dividedly derive a rear wheel driving force from the output of an HST for driving the front wheels. JP-A No. 66749/1997, for example, discloses such a transmission mechanism. With reference to FIG. 20 showing such a conventional transmission mechanism as is disclosed in this publication, a vehicle comprising a differential gear unit 1140, HST 1120 and drive source 1110 which are arranged from the front of the vehicle rearward has a transmission 1130, in which a driving force from the drive source is divided into a front wheel driving force and a rear wheel driving force. This construction has the following drawbacks. Incidentally, indicated at 1131 in the drawing is a front wheel drive shaft which is coupled to the differential gear unit 1140 for driving front axles 1151, and at 1132 is a rear wheel drive shaft which is connected to a rear axle 1152 by an unillustrated transmission shaft.
The transmission mechanism shown in FIG. 20 essentially requires the transmission 1130, which increases the number of parts and the cost. The provision of the transmission 1130 further entails the problem of impairing the reliability of the vehicle. Since the transmission 1130 serves as a power transmission path for both the front wheel driving force and the rear wheel driving force, the components of the transmission are burdened with a great load. Accordingly, the gears and other components are susceptible to damage, which lowers the reliability of the vehicle. Furthermore, the transmission 1130 is provided in a limited space between the differential gear unit 1140 and the HST 1130, necessitating difficult installation work, which leads to the problem that the vehicle is inefficient to assemble in its entirety.
With work vehicles equipped with a work device externally of the vehicle body, there is a need to provide a considerable distance between the engine and the HST because it is necessary to interconnect the drive source and the HST by means of universal joints. Stated more specifically, the drive source is installed on the vehicle body with rubber vibration isolators interposed therebetween, while the HST is connected to the differential gear casing which is fixed to the front axle cases. A vibration difference therefore occurs between the drive source and the HST. Accordingly, to interconnect the two components by a shaft with the vibration difference absorbed, universal joints are used for the shaft to be connected between the drive source and the HST. For this reason, there is a need to provide a distance permitting the arrangement of two universal joints in series between the HST and the drive source.
With vehicles wherein a PTO unit is provided which has a PTO shaft for a mower or like work device, and a hydraulic clutch for effecting or interrupting the transmission of the output of the PTO shaft, it is desired to position the PTO shaft inwardly of the vehicle body to the greatest possible extent while shortening the distance between the hydraulic clutch and a charge pump for supplying a hydraulic fluid to the clutch.
A shorter distance between the pump and the clutch is desirable because of the difficulty to be encountered in hydraulically controlling the clutch if the distance is great. Inward positioning of the PTO shaft is desirable in order to reduce the overall length of the vehicle including the mower or the like while horizontally positioning the connecting rod between the shaft and the mower or the like.
This will be described with reference to a front mount mower tractor which is equipped with a mower in front of the vehicle body. If it is attempted to shorten the overall length of the vehicle including the mower in an arrangement wherein the PTO shaft is caused to project forward from the front axle case, the transmission shaft interconnecting the PTO shaft and the mower must be given a great inclination. The transmission shaft is pivotally moved vertically when the mower is raised or lowered. If the inclination of the transmission shaft is greater, the angle through which the shaft is pivotally moved also increases, permitting the transmission shaft joints to produce a louder noise and impairing the durability of the joints. It is therefore desired that the PTO shaft be disposed inward longitudinally of the vehicle body (toward the rear of the vehicle body in the case of the front mount mower tractor) in order to shorten the overall length of the vehicle while obviating these drawbacks.
Thus, it is desired that the vehicle wherein the power of the drive source is transmitted to the travel device and to the PTO unit via the HST fulfill the foregoing requirements, whereas transmission mechanisms still remain to be developed which meet all of these requirements.
Conventionally, the following construction has been adopted for the pressure oil supply system for supplying a hydraulic fluid to power steering hydraulic devices or to hydraulic devices for lifting work devices such as mowers. The conventional pressure oil supply system has an arrangement wherein a hydraulic pump for supplying the pressure oil is attached to the drive source of the vehicle, and the casing of the differential gear unit is used also as an oil tank serving as the oil supply source for the hydraulic pump. The pump shaft of the hydraulic pump can be coupled to the drive source easily, while the use of the casing eliminates the cost increase and the increase of the vehicle size that would result if an oil tank for specific use is provided.
However, this arrangement requires an elongated inflow line interconnecting the hydraulic pump and the casing, consequently offering the piping increased resistance which is liable to impair the operation efficiency of the pump. Although the inflow line can be given an increased diameter to reduce the piping resistance, this will entail an increased cost.
The drive source is installed in the vehicle body with use of rubber vibration isolators, while the casing is fixed to the axle case, so that a vibration difference occurs between the hydraulic pump attached to the drive source and the casing. This results in the necessity of using a flexible tube or like pipe which is capable of absorbing the vibration difference between the pump and the casing for the inflow line for holding these two components in communication, hence a higher cost.
In order to shorten the distance between the hydraulic pump and the oil tank, it is possible to install a specific tank in the vehicle, as positioned in a vacant space in the vicinity of the drive source, but the installation of the specific tank leads to a higher cost and a greater vehicle size. Use of such a tank still involves the problem of vibration difference between the oil tank and the hydraulic pump.